A large number of single vertical bore oil wells exist in mature or maturing oil fields where the use of multilateral junctions in the vertical bores would allow additional reserves of oil or gas to be accessed. In areas where surface locations are limited, for example, in offshore drilling operations or drilling on the North Slope of Alaska, a multilateral junction from an existing wellbore is desirable however, cost often proves to be a limiting factor in the incorporation of multilateral junctions into the existing wellbores.
Conventional wellbores typically comprise a casing of either steel or concrete and a tubing string concentrically positioned therein, through which oil and gas are removed from subsurface reservoirs.
In one prior art application, the incorporation of a multilateral junction into an existing wellbore involves the removal of the tubing string within the wellbore to allow full bore access to the interior surface of the casing to create exit windows in the casing for lateral drilling operations. Such removal of the tubing string is an expensive and laborious undertaking.
In another prior art application, where the multilateral junction is to be installed at a location below the depth of a terminus of the original tubing string, the tools to be used to create the multilateral junction must be run through the smaller ID tubing and then must be used in the larger ID casing. In such an instance, the centralization of tools and the ability to retrieve the tools through the narrower tubing become issues.
A through-tubing multilateral system and method for installing the same for downhole oil drilling operations includes a tubing extension positioned in a downhole end of a tubing string in a wellbore and anchored in place. The tubing extension is dimensioned to obtain the most minimal tubing restriction possible such that it facilitates the installation of a multilateral junction therethrough.
The tubing extension of the through-tubing multilateral system includes a main body portion and thin walled section. The thin walled section is attached to an uphole edge of the body portion. The thickness of the wall of the thin walled section is less than the thickness of the wall of the body portion in order to allow for a lesser reduction in the ID of the string at the juncture between the original tubing string and the extension tubing. The tubing extension overall has an outside diameter less than an inside diameter of the tubing string (and any restrictions in the original tubing string) and is installed in direct contact with an inner surface of the downhole end of the tubing string. The juncture between the thin walled section and the tubing string is swaged to smooth the intersection between the original tubing string and the extension string.
The extension tubing string is anchorable by cementing the annulus or installing an inflatable or collapsible packer or similar device.
One advantage of this system and process is that only one set of equipment is needed for a particular size of tubing string. The tools used for each particular size of tubing string are, therefore, independent of the bore diameter defined by the interior surface of the casing. Another advantage of the system is its ability to enable the multilateral junction to be installed from within the tubing string rather than in the wider area of the casing below the tubing string. In addition to the ease of working within the tubing string as opposed to below the downhole end of the tubing string, the system offers considerable savings over removing the tubing string from the wellbore and installing a multilateral junction in a conventional manner, especially in remote locations.